1. Field of the Invention
The present invention relates to a driving circuit of a liquid crystal display panel, and more particularly to a driving circuit, in which the driver IC chips each obtain an approximately identical input voltage.
2. Description of the Prior Art
A thin film transistor liquid crystal display (TFT-LCD) panel utilizes many thin film transistors (TFTs), in conjunction with other elements such as capacitors and bonding pads, arranged in a matrix as switches for driving liquid crystal molecules to produce brilliant images. Generally, the conventional TFT-LCD panel includes an upper substrate having a color filter, a lower substrate, and liquid crystal materials between the substrates. The lower substrate comprises a plurality of scan lines (gate lines) and a plurality of signal lines (source lines) orthogonally cross over the scan lines. At least one TFT is located near the crossover of the scan line and the signal line, as a switch device for the pixel.
Please refer to FIG. 1. FIG. 1 is a schematic diagram showing a structure of a conventional liquid crystal display panel. As shown in FIG. 1, a TFT-LCD panel 10 comprises a substrate 12 and an X-printing wiring board 14. In addition, the TFT-LCD panel 10 comprises a plurality of flexible printing circuit boards (FPC) 16 for electrically connecting the X-printing wiring board 14 and the substrate 12. Source driver IC chips 18 are positioned on the side region of the substrate 12 connected to the FPC 16 and electrically connecting to the FPC 16.
A plurality of scanning lines S1, S2, . . . , and Sm and a plurality of signal lines D1, D2, . . . , and Dn are positioned on the substrate 12. The scanning lines S1, S2, . . . , and Sm orthogonally cross over the signal lines D1, D2, . . . , and Dn to define a pixel matrix (not shown) in an active region 19 on the substrate 12. In addition, the substrate 12 further comprises an outer lead bonding region (OLB) 20 and a gate driving circuit 22 positioned in the OLB 20. The gate driving circuit 22 comprises driver IC chips 22a and 22b, for outputting switch/addressing signals to the scanning lines S1, S2, . . . , and Sm. The source driver IC chips 18 are used for outputting image data signals to the signal lines D1, D2, . . . , and Dn. The driver IC chips 18, 22a, and 22b are formed on the surface of the substrate 12 by chip-on-glass (COG) technology. The driving circuit 22 comprises a plurality of conductive wires 24 for electrically connecting the driver IC chips 22a and 22b in series. The conductive wires 24 are formed directly on the surface of the substrate 12, and such design is called wiring on array (WOA).
When the liquid crystal display panel 10 is operated, a driving voltage for a controlling signal 28 is output from the X-printing wiring board 14, as shown in FIG. 1, passes through the FPC 16 and the conductive wires 24, and inputs to the driver IC chips 22a and 22b. Finally, the driver IC chips 22a and 22b output switch/addressing signals to the scanning lines S1, S2, . . . , and Sm according to the input voltage. In addition, as shown in FIG. 2, the driver IC chips 22a and 22b are electrically connected in series. Since the conductive wires 24 produce resistances R1 and R5, the driver IC chip 22a has an inner resistance R3, and contact points of the elements produce resistances R2, R4, and R6, there is a relatively large total resistance.
The width of the conductive wires is broadened as wide as possible to reduce the resistance in traditional wiring techniques. The conductive wires connect ICs from the first IC to the last IC in series. However, the interface impedance between metal lines and ITO layer may be as high as 200 Ω. Thus, the difference of the wiring resistance between the first IC and the wiring resistance of the last IC will be as high as 500 Ω. Therefore, the driver IC chips 22a and 22b receive different input voltages when a voltage of the controlling signal 28 is applied to them, and in turn the output voltages from the driver IC chips 22a and 22b are different. The received voltage difference between the first IC and the last IC may be about 0.3V which leads the liquid crystal display panel 10 to have a band mura problem and an uneven brightness, resulting a poor display quality. As shown in FIG. 3, the display 30 is schematically shown to have a band mura.
Please refer to FIG. 4 illustrating a well known method to inhibit the band mura by adding a resistor 31 at the signal input position on the printed circuit board, such that an input signal would have an oscillating distortion with an amplitude larger than the voltage difference caused by the wiring impedance to obscure the band mura phenomenon. However, the power consumption of the entire driving circuit would be increased in such method.
Therefore, a good driving circuit of a liquid crystal display panel is still needed for giving each driver IC chip an approximately identical input voltage to avoid band mura phenomenon.